Safety control and ignition system for fuel burners



Oct. 25, 1949.,

C. K. STROBEL SAFETY CONTROL AND IGNITION SYSTEM FOR FUEL BURNERS 2 Sheets-Sheet 1 INVENTOR TTORNEY Filed Dec. 19, 1946 (*lzarlar If. Szzobel HAS; A

Oct. 25, 1949.

'c. S'TROBEL SAFETY CONTROL AND IGNITION SYSTEM FOR- FUEL BURNERS Filed Dec. 19, 1946 2 Sheets-Sheet 2 B Y 11g ATTORNEY Patented Oct. 25,

SAFETY CONTROL AND IGNITION SYSTEM FOR FUEL BURN ERS Charles K. Strobe], Pittsburgh, Pa., assignor to Robertshaw-Fulton Controls Company, a corporation of Delaware Application December 19, 1946, Serial No. 717,183

14 Claims. (Cl. 158117.1)

This invention relates to control systems for fuel burning apparatus and, more particularly, to electrically operated safety control and ignition systems.

Gaseous fuel burners have been controlled by 5 a controlling the flow of fuel to the main burner means responsive to the presence of a flame at l0 and may take the form of a solenoid valve the burner so that whenever the flame is eX- 24 normally biased to closed position by gravity, tinguished the supply of fuel is automatically springs or the like to prevent the supply of fuel cut-off. In order to start the burner in operation to the main burner Ill. The valve 24 has an enit is necessary to supply fuel thereto while the ergizing coil 26 operatively associated therewith flame is non-existent. The common method of for causing it to open when energy of sumcient providing for such supply is by the use of a timvalue is supplied to the coil 26. As will be aping device which operates during a limited period parent, the supply of fuel to the main burner to permit the supply irrespective of the condi- I0 is under control of both the main fuel cock tion of the flame responsive means. At th end I4 and the solenoid valve 24 while the supply of of the timin period the fuel supply is again cutfuel to the pilot burner I 8 is under control of oil unless the flame responsive means has bethe main fuel cock I4 only. come responsive to a flame at the burner. A control device comprising a first relay 28 is An object of this invention is to simplify exemployed for controlling the operation of the isting systems by eliminating special timers and valve 24 by controlling the energization of the utilizing equivalent characteristics inherent in a oi 26. T is first relay 28 includes a coil 30 and simple control device of the electronic type. the usual control elements including a movable Another object of the invention is to adapt armature means 32 adapted to engage a fixed the system for electronic control to effect the contact means 34 when the coil 3llis supplied timing function and also improve the general with energy of suflic'ient value.v The relay coil safety features thereof. has a capacitor 36 of suitable size connected Another object of the invention is to preserve in parallel circuit therewith .to smooth the curexisting parts of the safety control system which rent flow in the coil30. I have proved reliable while improving the per- A second relay 38 is provided for controlling formance thereof in the modified system. 30 the igniting means 22 and includes a coil, 40 and Other objects and advantages will become apa pair of movable armature means 42 and 44. parent from the following description taken in The armature means 42 and 44 are normally connection with the accompanying drawings, biased into engagement with contact means 46 wherein: and 48 respectively. Upon energy of sufiicient Fig. l is a schematic view of the improved con- 35, value being supplied to the coil 40, the armature trol system for controlling the flow of fuel to a means 42 and 44 become disengaged from the fuel burning apparatus; and contact means 46 and 48 and the armature means Fig. 2 is a similar schematic view of a modifi- 42 thereby becomes engaged with contact means cation of the improved control system. 50. I

Referring more particularly to Fig. 1 of the 40 An electronic device 52 of the triode type is drawings, the main burner 10 is shown as being provided for controlling the operation of the supplied with fuel from a main fuel pipe l2 under relays 28 and 3B. The cathode 54 of this device control of a main fuel valve or cook l4 which, .52 is indirectly heated by a heating element 56 if desired, may also incorporate a thermostatic and these elements are housed, together with device (not shown) responsive to temperatures the control grid 58 and anode 60, within the caused by operation of the main burner to. The usual envelope 62. main fuel cock l4 also includes a main switch A step-up, step-down transformer 64 controls It which is operated to closed position when the the energization of the electronic device 52 as main fuel cock is opened and is normally open well as the various other electrical elements rewhen the main fuel cock is closed. ferred to. The transformer 164 comprises a line A pilot burner I8 is located in proximity of voltage primary 66, a hi h voltag s dary the main burner In to ignite the fuel flowing 68 and low voltage secondaries 1a and 12. The therefrom and is supplied with'fuel by a conduit line voltage primary 66 is connected at one end 20 under control of the main fuel cock l4. Electhrough the main switch I6 to a line wire 14 and tric igniting means 22, preferably in the form at the opposite end to a line wire 16. While any of a coil of resistance wire, is located in lightingproximity to the pilot burner l8 to ignite the fuel issuing therefrom.

Electrically operable means is provided for 3 suitable voltage may be employed, depending on the voltage of the power source available and the operating characteristics of the electronic device 52, the primary winding 66 in this instance is connected to a 110-volt source of energy and the windings 68, and 12 may be designed to operate at voltages of 250 volts, 6 volts and 4 volts, respectively. In this preferred embodiment of the invention the transformer 64 constitutes the source of energy and is so named for the purposes of description and definition, although some primary source of energy must be connected therewith as has been described. As

far as the present invention is concerned, the

construction of the transformer 64 and the connections for supplying energy at various potentials to the control system can be varied, it being understood that the step-up winding may be omitted if an appropriate electronic device is used and connected directly to a suitable primary power source.

As previously indicated, the safety control system to which this invention pertains includes flame responsive means of a known type'which operates in conjunction with cooperating elements to control the flow of fuel to the main burner l0 should the pilot burner l8 become extinguished. Thus, the flame responsive means comprises a variable resistor 18 in the form of a coil of wire of material having normally a relatively low resistance and a positive temperature coeflicient of resistance. The resistor 18 is located in proximity of a flame from the pilot burner l8 and is adapted to increase appreciably in resistance value when heated thereby. It will be apparent, however, that the passage of electrical energy through the resistor 18 will also cause it to become heated to some extent but it is the heating thereof by the flame of the pilot burner i8 which is primarily responsible for its usefulness in the disclosed system.

A second variable resistor 80 is provided for cooperation with the resistor 18 and is also formed from a coil of wire having normally a relatively low resistance and a positive temperature coeflicient of resistance. This second resistor 80 is not, however, exposed to the flame of any burner and is adapted to vary its resistance only due to the passage of electrical energy therethrough.

A potential divider 82 is provided for cooperation with the resistors 18 and 80 in the operation of the electronic device 52. The potential divider 82 consists of the usual resistance means 83 and 85 in series and provided with a tap 84 forming part of a connection between the circuit of the secondary l0 and that of the secondary 68 as will hereinafter be apparent.

A timing circuit of conventional form is provided for producing the necessary time delay for operation of the system. This timing circuit comprises a resistor 86 and a capacitor 88 in parallel and connected to the circuit of the grid 58 of the electronic device 52. The various elements of the system are completed by the provision of a resistor 9|! for adjusting the value of the energy supplied to the igniting means 22.

In the subsequent description of the operation of the system the various circuit connections for the described elements will be outlined together with the cooperation between the various elements to produce the required results. Generally, the resistor 18 cooperates with the resistor 80 and the potential divider 82 to control the grid bias of the electronic device 52 in response to flame conditions at the pilot burner l8. The

control elements of the relay canfunction to control the operation of the igniting'means 22 and the solenoid fuel valve 24. It should be noted that the electronic device 52 acts as its own rectifler and no separate D.-C. supply is necessary for operation of the system.

With the various elements in the position shown in Fig. l of the drawings and the system inoperative, the fuel supply and the 110-volt supply to the primary E6 of the transformer 64 are turned on simultaneously by operation of the main fuel valve 14. Thus, fuel will flow in the conduit to the pilot burner l8 and the transformer 64 will be energized.

As the solenoid valve 24 is biased closed there is no flow of fuel in the main fuel pipe l2 to the main burner ID at this time. In accordance with the known characteristics of electronic de vices,.there is a time oelay before any energy flows from the output circuit of the device until the heating element 56 reaches operating temperature and the circuit potentials are stabilized. When the electron flow in the device 52 is established there is an initial output current which is insufficient for operation of the system so that the movable circuit elements, such as armatures 42 and 44, remain in the biased positions shown in Fig. 1. This holding of the energy in the output circuit below the effective energizing value is due .to a negative bias being applied to the grid 58 as'long as the resistor 18 is unheated by a flame from the pilot burner l8.

Thus, on one-half of the alternating current cycle the anode potential is positive with respect to the cathode 54 and there is an electron flow between the cathode 54 and the anode 60. On the succeeding'half-cycle, the anode potential becomes negative with respect to the cathode potential and no electron flow occurs between these elements. During this second half-cycle the capacitor 36 supplies energy to the coil of the relay 28. However, when the anode potential is positive as described the electron flow depends upon the .amount and sign of the potential at the grid 56. As this grid potential changes in a negative direction with respect to the potential of the cathode 54, the electron flow to the anode 60 is reduced.

Under the circumstances described, the cathode 54 becomes heated by the heating element 56 of the control device 52 which is connected to the 6-volt secondary 10 of the transformer 64 by a wire 92 from one terminal of the secondary 10 to one end of the heating element 55 and b.\ a wire 94 from the other terminal of the secondary 10 to the opposite end of the heating element 56. Meanwhile, the igniting means 22 i: energized through a circuit from the lower terminal of secondary 12, wire 96, igniter 22, wire 98, resistor 90, wire I00, contact means 48, arma-- ture means 44 and wire I02 to the upper ter-- minal of the transformer secondary 12. The fuel issuing from the pilot burner I8 is thus isnited and the flame produced thereby serves to heat the resistor 18 and cause it to increase in resistance value.

The heating and increase in resistance value of the resistor 18 is effective for causing the desired shift in the biasing of the input circuit of the electronic device 52 sufficiently to render the same conductive. The input circuit of the electronic device 52, comprising control grid 56 of the foregoing described the igniting means 22 terminal of transformer secondary I2,

and cathode 54, is connected as the detector branch of a bridge circuit in which the resistors I8 and 80 and resistance means 83. 85 of p tential divider 82 each form one of the other four branches. This circuit is connected across the 6-volt supply as follows: Upper terminal of secondary I0, wire 92, bridge junction A, resistance means 83, junction B, resistance means 85, junction C, resistor 80, Junction D and resistor I8 back to Junction A, it being noted that wire 94 completes the connection of junction back to the lower terminal of secondary I0. The detector branch of this bridge is connected at junction B by tap 84 and wire I04 to cathode 54, grid 58, resistor 86 and capacitor 88 in parallel and wire I06 to junction D.

The lower terminal of the 250-volt secondary 68 is connected to the wire I04 to cathode 54 by a wire I08 and the upper-terminal of secondary 58 is connected by wire IIO to one terminal of the relay coil 30, the opposite terminal of which is connected by a wire I I2 to the anode 60. The capacitor 36 in parallel circuit with the relay coil 30 serves to smooth the half-wave rectified current of this outut circuit of the electronic device 52, as will be apparent. It is further apparent that the current in the 6-volt secondary 10 be approximately 180 degrees out of phase with the current in the 250-volt secondary 60.

When the resistor I8 is suiilciently heated by the flame from the pilot burner I8, the elements bridge'cincuit cooperate to shift the bias of the grid 58 in a positive direction sufficiently to increase the output current of the electronic device 52 above that required to eflectively energize the relay coil 30. The relay armature means 32 then engages contact means 34. A circuit for the solenoid valve 24 is then established as follows: lower terminal of transformer secondary I2 to wire 96, wire II4, solenoid valve 26, wire 'I I6, contact means 34, armature means 32, wire H8 and wire, I 02 back to the upper terminal of the transformer secondary 12. The solenoid valve 24 is thus actuated to open position to supply fuel to the main burner I0 where it is ignited by the flame from the pilot burner l8.

It is apparent that once the main burner I0 is in operation, it is no longer necessary for the igniting means 22 to be energized. Consequently, means are provided to automatically deenergize at this time. This igniter deenergizing means includes a lock-out circuit for the second relay 38. Thus, when relay 28 is energized and engagement between armature means 32 and contact means 34occurs,

the second relay 38- becomes energized through a circuit which may be traced as follows: upper wire II8, armature 32, contact means 34;;wire IIB, wire I20, contact means 46, armature 42, second relay coil 40, wire to the lower terminal of ondairy I2. will serve to cause the armature means 42 and 44 of the second relay 38 to move out of engagement with the contact means 46 and 48, respectively, and the armature means 42 will thus be moved into engagement with the contact means 50. In the latter event, the circuit of the igniting means 22 will be opened and the energization of the igniting means 22 will cease. The locking arrangement for the auxiliary relay 38 is now effective to maintain the coil 40 enersized until the system is manually shut-oil by the transformer secwire I02.

I22, and wire SG'back';

The establishment of this circuit" '-I34 with which the strip I30 operation of the main switch I6. The circuit now established by the foregoing operation of the second relay 38 may be traced as follows: from the upper terminal of transformer secondary 12, wire I02, wire I24, armature means 42, relay coil 40, wire I22 and wire 96 back to the lower terminal of the transformer secondary I2.

The system will remain in the steady state operation described. until manually shut-off by operation of the main fuel valve I4 or until the flame at the pilot burner I8 is extinguished. In the latter case, the potential-drop across the resistor I8 is decreased as soon as this element starts to cool. The rate of decrease of this potential is augmented by the action of the resistor which is also responsive to temperature changes therein, as described. The bias of the grid 58 in finally changed in a negative direction so as to cause the output current of the electronic device 52 to decrease suiiiciently to cause effective deenergization of the relay coil 30. The armature means 32 then moves under its bias out of engagement with the contact means 34 and the system returns to its initial condition with the solenoid valve 24 closed to shut-oil fuel flowing to the main burner I0. As previously described, the second relay 38 remains in energized condition to maintain the circuit to the igniting means 22 open. Consequently, the system must be manually restarted by manipulation of the main fuel valve- I4 and is not recycling.

If a rapid on-ofi-on operation of the main fuel valve I4 is conducted when starting, or if a rapid off-on operation is made during steady state running, then there is a substantial delay in rendering the electronic device 52 conductive due to the time required to stabilize the circuit potentials. Hence, the starting conditions appropriate for systems of this nature are provided for.

feature is of importance in recycling systems inasmuch as a purge period for dissipation of un- -burned fuel at the burner should be provided before another igniting operation is established.

Thev thermal switch I28 in this embodiment comprises a bimetal strip I30 having a heater coil I32. associated therewith and contact means engages when the heater coil I32 is deenergized. As the remaining elements of the system disclosed in Fig. 2 are identical with those disclosed in the embodiment of Fig. 1,-further description of the parts is considered unnecessary.

In the operation of the embodiment shown in Fig. 2, the closing of the main switch I 6 by operation of the fuel valve I4 serves to energize the igniting means 22 through the following circuit:

contact means 50,

conjunction with from lower terminal of transformer secondary I2. wire I36, armature means 32, contact means I26, wire I38, resistor 90, wire 98, igniting means 22, wire I40, bimetal strip I30, contact means I34 and wire I42 back to the upper terminal of transformer secondary I2.

After the resulting flame from the pilot burner I8 has heated the resistor I sufllciently to render the electronic device 52 conductive, as previously explained, the coil 30 of the relay 28 becomes effectively energized. The armature means 32 then moves into engagement with the contact means 34 and opens the circuit of the igniting means 22 by its consequent disengagement from contact means I26. A circuit is then established for the solenoid valve 24 and also for the heater coil I32 as follows: from the upper terminal of the transformer secondary I2, wire I42, heater coil I32, wire I44, relay coil 26, wire IIG, contact means 34, armature means 32 and wire I36 back to the lower terminal of transformer secondary I2.

Upon establishment of the foregoing circuit, the solenoid valve 24 is caused to open and admit fuel to the main burner I0 where it is ignited by the flame from the pilot burner I8. Moreover, the heater coil I32 becomes energized and causes the bimetal strip I30 to warp and disengage itself from the contact means I34. The apparatus is then in steady state operation until the fuel supply is shut-off by operation of main valve I4 or the pilot burner I8 becomes extinguished.

Upon extinguishment of the pilot burner I8, the resistor I8 will cool and cause a shift in the grid bias of the electronic device 52 as previously described and the relay 28 will operate to open the circuit of valve coil 26 and cause the solenoid valve 24 to close. Such action will serve to engage the armature means 32 of the relay 28 with its contact means I26 but, as the bimetal strip I30 is disengaged from the contact means I34, the igniting means I22 will not immediately be energized. It is apparent that the thermal switch I28 will be so designed that its cooling period is longer than that of the resistor 18 so as to allow a suitable period between the time of closing of the solenoid valve 24 and complete reenergizing of the igniting means 22 to permit any unburned fuel from the main burner ID to flow away.

When the heater coil I22 and the bimetal strip I30 cool off sufficiently to cause engagement of the strip I30 with the contact means I34, then the igniting means 22 will again be energized through the described igniter circuit. Another cycle of the system would thus be initiated and the sequence of operation would be as previously described.

While a preferred embodiment of the invention and a modification thereof have been shown and described, it is apparent that many other modifications will occur to those skilled in the art. For example, the separate 4-volt secondary has been employed to indicate that the power consumption may be reduced by this expedient instead of using the 6-volt secondary for all the low voltage circuits. Hence, the invention is not to be deemed limited to the form shown and described or otherwise than as defined by the scope of the claims appended hereto.

I claim:

1. In a control system for fuel burning apparatus having a source of fuel supply, the combination of electrically operable means movable between positions for controlling the fuel supply to be burned, an electronic device having the output circuit thereof connected to said means for supplying operative energy thereto, a bridge circuit comprising a branch connected to the input circuit of said device, and a plurality of resistance means in at least two other branches of said bridge, one said resistance means being responsive to temperature changes caused by the burning fuel for varying in resistance value and being cooperable with the other said resistance meansfor varying the potential across said input circuit and causing variations in the output energy supplied to said electrically operable means sufficient for movement thereof between said positions.

2. In a control system for fuel burning apparatus having a source of fuel supply, the combination of electrically operable means movable between positions for controlling the fuel supply to be burned, an electronic device having the output circuit thereof connected to said means for supplying operative energy thereto, a bridge circuit comprising a branch connected to the input circuit of said device, and a plurality of resistance means in at least two other branches of said bridge, one said resistance means being responsive to temperature changes caused by the burning fuel and another said resistance means being responsive to the passage of energy therethrough for varying in resistance value, said plurality of resistance means being cooperable for varying the potential across said input circuit and causing variations in the output energy supplied to said electrically operable means suflicient for movement thereof between said positions.

3. In a control system for fuel burning apparatus having a source of fuel supply, the combination of electrically operable means movable between positions for controlling the fuel supply tobe burned and including an energizing coil, an electronic device having the output circuit thereof connected to said coil for supplying operative energy thereto, a bridge circuit comprising a detector branch connected to the input circuit of said device, and a plurality of resistance means in at least two other branches of said bridge, one said resistance means being responsive to temperature changes caused by the burning fuel for varying in resistance value and being cooperable with the other said resistance means for varying the potential across said input circuit and causing variations in the output energy supplied to said coil suificient for movement of said electrically operable means between said positions.

4. In a control system for fuel burning apparatus having a source of fuel supply, the combination of electrically operable means movable between positions for controlling the fuel supply to be burned and including an energizing coil, an electronic device having the output circuit thereof connected to said coil for supplying operative energy thereto, a bridge circuit comprising a detector branch connected to the input circuit of said device, and a plurality of resistance means in at least two other branches of said bridge, one said resistance means being responsive to temperature changes caused by the burning fuel and another said resistance means being responsive to the passage of energy therethrough for varying in resistance value, said plurality of resistance means being cooperable for varying the potential across said input circuit and causing variations in the output energy supplied to said coil sufficient for movement of said electrically operable means between said positions.

5. In a control system for fuel burning apparatus having a source of fuel supply, the combination of electrically operable means movable between positions for controlling the fuel supply to. be burned and including an energizing coil, an electronic device having the output circuit thereoi connected to said coil for supplying operative energy thereto, a bridge circuit comprising a detector branch connected to the input circuit of said device, and a plurality of resistance means in the other branches of said bridge, said resistance means including potential divider means and a pair of variable resistors, one said resistor being responsive to temperature changes caused by the burning fuel and the other said resistor being responsive to the passage of energy therethrough for varying in resistance value, said plurality of resistance means being cooperable for varying the potential across said input cir- V cult and causing variations in the output energy supplied to said coil suflicient for movement of said electrically operable means between said positions.

6. In a control system for fuel burning apparatus having a source of fuel supply, the combination of electrically operable means for controlling the fuel supply to be burned and including a relay coil and armature means operable thereby between difierent positions relative to cooperating contact means, an electronic device having the output circuit thereof connected to said coil for supplying energy thereto, electric igniting means for the fuel having an energizing circuit controlled by cooperation between said armature means and said contact means, a bridge circuit comprising a branch connected to the input circult of said device, and a plurality of resistance means in at least two other branches of said bridge, one said resistance means being respon-' sive to temperature changes caused by the burning fuel for varying in resistance value and being cooperable with the other said resistance means for varying the potential across said input circuit and causing variations in the output energy supplied to said coil sufllcient for operation of said armature between said positions, said igniting means being energized in one thermal condition of said one resistance means and being deenergizedin another said condition thereof.

7. In a control system for fuel burning appara tus having a source of fuel supply and main and pilot burners, the combination of electrically operable means for controlling the fuel supply to the main burner, a control device including a, relay coil and one set of contact means disposed to be opened and another set of contact means, disposed to be closed upon energization of said coil, an electronic device having an input circuit and an output circuit, a source of electrical energy, electric igniting means for the pilot burner, means connecting said output circuit, said source, and said-coil, means for biasing said input circuit so as to hold the energy in said output circuit below the value necessary to energize said coil, said last means including a resistance means responsive to temperature changes caused by operation of the pilot burner and adapted to vary in resistance value in one thermal condition to cause a shift in said biasing sufliciently to energize said coil, means for connecting said igniting means to said source and said one set of contacts for energization of said igniting means only 70 when said coil is deenergized, and means for connecting said electrically operable means to said source and said other set of contacts for energization of said electrically operable means only when said coil is ener 8. In a control system for fuel burning apparatus having a source of fuel supply and main and pilot burners, the combination of electrically operable means for controlling the fuel supply to the main burner, a control device including a relay coil and one set of contact means disposed to be opened and another set of contact means disposed to be closed upon energization of said coil, an electronic device having an input circuit and an output circuit, a plurality of sources of alternating energy, electric igniting means for the pilot burner, means connecting one of the sources of energy to said output circuit and said coil, means for biasing said input circuit so as to hold the energy in said output circuit below the value necessaryto energize said coil, said last means including a resistance means responsive to temperature changes caused by operation of the pilot burner and adapted to vary in resistance value in one thermal condition to cause a shift in said biasing to increase the output energy sufliciently to energize said coil, connections between a second of said sources of energy and said last means, and connections between a third of said sources of energy, said igniting means and said electrically operable means, said last connections including both one and the other sets of said contact means for energization of said igniting means and said electrically operable means alternatively.

9. In a control system for fuel burning apparatus having a source of fuel supply, the combination of electrically operable means for controlling the fuel supply to be burned and including an energizing coil, an electronic device having a cathode, anode and control grid, a source of electrical energy, a bridge circuit having opposite junctions connected to 'said source, a detector branch of said bridge having one terminal connected to said cathode and the opposite terminal connected to said grid, said one terminal being also connected to said source, said coil and said anode in series, said detector branch normally maintaining said control grid at a potential such that said coil is effectivel deenergized, and a plurality of resistance means in the other branches of said bridge including a pair of variable resistors, sive to temperature changes caused by the burning fuel for varying in resistance value and cooperating with the other said resistance means to maintain said control grid at a diilerent potential such that said coil is effectively energized whenever said one-resistor is in one thermal condition.

it). In 'a control system for fuel burning apand pilot burners, the combination of electrically operable means for controlling the fuel supply to the main burner, a control device including a relay coil and one set of contact meansdisposed to be opened and another set'of contact means disposed to be closed upon energization of said coil, an electronic device having a cathode, anode and control grid, "an electric igniter for the pilot burner, a, source of electrical energy, a bridge circuit having opposite junctions connected to said source, a detector branch of said bridge having one terminal connected to said cathode and the opposite terminal connected to said grid, Said one terminal-being also connected to said source, said coil and said anode in series, said detector branch normally maintaining said control grid at a potential such that said coil is effectively deenergized, a plurality ofresisone said resistor being responis in one thermal condition, and means for connecting said igniter and said electrically operable means to said source including both one and the other sets of said contact means for energization of said igniter and said electrically operable means alternatively.

11. In a safety control for fuel burning apparatus having main and pilot burners and means for supplying fuel thereto, comprising electrically operable means responsive to variations in the current value supplied thereto for controlling the supply of fuel for operation of the main burner, an electronic device having the output circuit thereof connected to said means for supplying current thereto, electric igniting means for the pilot burner controlled by said electrically operable means for energization only when the main burner is not operating, means for initially biasing the input circuit of said device so as to hold the current value in said output circuit below that necessary to cause operation of the main burner, said last means including means responsive to a flame at the pilot burner to cause a shift in said biasing sufliciently to cause operation of the main burner, and means electrically connected to said igniting means for delaying energization thereof when said initial biasing is resumed upon failure of the flame at the pilot burner.

12. In a safety control for fuel burning apparatus having main and pilot burners, comprising electrically operable means responsive to variations in the current value supplied thereto for controlling the operation of the main burner, an electronic device having the output circuit thereof connected to said means for supplying current thereto, electric igniting means for the pilot burner controlled by said electrically operable means for energization only when the main burner is not operating, means for initially biasing the input circuit of said device so as to hold the current value in said output circuit below that necessary to cause operation of the main burner, said means including a thermal resistor responsive to a flame at the pilot burner for varying in resistance value to cause a shift in said biasing sufficiently to cause operation of the main burner, and thermally responsive means having a cooling period longer than that of said thermal resistor electrically connected to said igniting means and controlled by said electrically operable means, said thermally responsive means being effective for delaying energization of said igniting means when said initial biasing is re-.

sumed upon failure of the flame at the pilot burner.

13. In a control system for fuel burning apparatus having a source of fuel supply and main and pilot burners, the combination of electrically operable means for controlling the fuel supply to the main burner, a first relay for controlling the operation of said electrically operable means, an electronic device, a second relay adapted to be controlled by said first relay, igniting means for the pilot burner controlled by said second named relay, a transformer having a pluralityof secondary windings, connections between one of said secondary windings, said first-named relay and said electronic device constituting the output circuit of said electronic device and controlling the energization of said first-named relay, resistance means connected to a control grid of said electronic device constituting with another of said windings the input circuit of said electronic device, said resistance means including flame responsive means adapted upon the presence of a flame at the pilot burner to shift the grid bias sufllciently to energize the first-named relay for operation of said electrically operable means, connections between another of said windings, said second-named relay and said igniting means to energize said igniting means prior to said operation of said electrically operable means, and further connections between another of said windings and said second named relay to energize said second-named relay independently of the firstnamed relay and maintaining said igniting means deenergized after said operation of said electrically operable means.

14. In a control system for fuel burning apparatus having a source of fuel supply and main and pilot burners, the combination of electrically operable means for controlling the fuel supply to the main burner, a first relay having control elements for controlling the operation of said electrically operable means, an electronic device, a second relay having control elements and connected to said first-named relay to be controlled thereby, igniting means for the pilot burner controlled by certain of the control elements of said second-named relay, a transformer having a plurality of secondary windings, connections between one of said secondary windings, said firstnamed relay and said electronic device constituting the output circuit of said electronic device and controlling the energizr ion of said firstnamed relayfor operation of its control elements, resistance means connected to a control grid of said electronic device constituting with another of said windings the input circuit of said electronic device, said resistance means including flame responsive means adapted upon the presence of a flame at the pilot burner to shift the grid bias sufliciently to energize the first-named relay for operation of said electrically operable means, connections between another of said windings, said certain control elements of said secondnamed relay and said igniting means to energize said igniting means prior to operation of said electrically operable means, and further connections between another of said windings, said second-named relay and other of its control elements to energize said second-named relay independently of the first-named relay and maintain said igniting means deenergized after said operation of said electrically operable means.

CHARLES K. STROBEL.

REFERENCES CITED The following references are of record in the file of this patent:

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